The present application relates to an organic electroluminescent device making use of an amine compound suited as an organic material for organic electroluminescent devices, and also to a display device making use of one or more of such organic electroluminescent devices.
As self-emitting flat panel displays of low power consumption, high response speed and no viewing angle dependency, display devices making use of organic electroluminescent devices (so-called organic EL devices) have been drawing attention in recent years.
In general, an organic electroluminescent device is provided with an organic layer between an anode and a cathode, and emits light as a result of recombination of holes and electrons injected from the anode and cathode, respectively, in the organic layer. Developed as such organic layers include, for example, a construction that a hole transport layer, a light-emitting layer with an organic light-emitting material contained therein, and an electron transport layer are stacked one over another from the side of the anode, and a construction that a light-emitting material is included in an electron transport layer to form an electron-transporting, light-emitting layer.
Organic electroluminescent devices of such a construction are self-emitting devices. When constructing a display device by using these organic electroluminescent devices, it is, therefore, most important requirements to provide them with a longer service life and to ensure their reliability. Accordingly, research is now under way on organic materials that make up organic electroluminescent devices.
A hole transport layer assumes an important role to control the balance of recombination in a light-emitting layer, and in addition, is required to have a structure excellent in thermal stability. N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD), a representative material known as a hole transport material, has a low glass transition point (Tg) as its thermal property although it is a good hole transport material.
In recent years, there have been disclosed organic materials excellent in amorphousness owing to the introduction of the thiophene nucleus into the central skeletons of amine compounds (Japanese Patent Laid-Open No. Hei 10-219242 and Japanese Patent Laid-Open No. 2003-13054). Further, compounds, each of which contains amino groups around one thiophene skeleton via aryl groups interposed between the amino groups and the thiophene skeleton, are disclosed to function as hole injection materials (Japanese Patent Laid-Open No. 2001-345181 and Japanese Patent Laid-Open No. 2003-267973).
As a cause of a deterioration in brightness as a result of drive of an organic electroluminescent device, the crystallization of an organic material due to joule heat associated with the drive is considered to be attributable. As a method adopted to resolve this problem, the molecular weight is increased to heighten the amorphous properties. For example, 4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine (m-MTDATA) the molecular skeleton of which is of the starburst type is known as a material excellent in amorphousness (Applied Physics Letters (U.S.A.), 65, 807-809 (1994)). However, further improvements are still desired because the use of this m-MTDATA requires a high voltage.
As a method for allowing an organic electroluminescent device to operate at a lower voltage, it is necessary, to provide the organic electroluminescent device with increased charge transport ability. With an oligothiophene which is an oligomer of six or more thiophene units linked together, the organic electroluminescent device is known to exhibit good charge transport ability as an organic semiconductor (Japanese Patent Laid-Open No. 2002-322173). Concerning an amine compound with three thiophene units linked together as a compound having such property introduced therein, characteristics of an organic electroluminescent device are disclosed, but a high voltage is still required to obtain brightness (Applied Physics Letters (U.S.A.), 70, 699-701 (1997)).